The present invention relates to water treatment, in particular to a process for the removal of inorganic contaminants from water.
Water supplies for drinking water often contain unacceptably high levels of inorganic ionic species. These inorganic ionic species include anions such as perchlorate, nitrate, bromide and bromate, other anions such as species containing arsenic V, arsenic III and chromium, as well as other inorganic anions known to the art to be removable by anion exchange agents; and inorganic cations such as ions of calcium, copper, nickel, magnesium, lead, mercury, cadmium and beryllium, and other inorganic ionic species known to the art to be removable by cation exchange agents.
Water containing such undesirable inorganic ionic species also often contains ions which compete with these undesirable inorganic ionic species for removal by ion exchange processes. Such competing ions include sulfate, silicate, nitrate, bicarbonate and dissolved organic carbon (DOC). Strong base anion exchange resins, such as the MIEX® Resin of Orica Australia Pty. Ltd. described in U.S. Pat. No. 5,900,146, which can be used to remove inorganic anions, typically have over six times the affinity for sulfate as for arsenate.
Conventional ion exchange resins have been used as water softeners to remove alkalinity from water in combination with purification treatments for removing inorganic contaminants. See, e.g., U.S. Pat. No. 4,839,057. A number of prior methods for removing inorganics from water using conventional ion exchange resins, e.g., as described in U.S. Pat. Nos. 5,149,437, 6,197,193, 6,203,705, and 5,494,582, have typically involved running the water through a column packed with the resin, or a resin bed. U.S. Pat. No. 6,200,471 discloses the use of a conventional ion exchange resin within a water bottle.
U.S. Pat. No. 5,595,666 discloses the use of a magnetic polymer resin for removal of actinides from contaminated water. The resins comprise polyamine-epichlorohydrin resin beads with ferrites attached to the surfaces of the beads, and contaminated water is flowed through a packed column of these beads.
U.S. Pat. No. 5,900,146 discloses magnetic and nonmagnetic ion exchange beads having solid particulate material, which is magnetic or responsive to magnetic force, distributed throughout the beads. PCT Publication WO 96/07615 discloses dispersing particulate magnetic ion exchange agent in water being treated for removal of dissolved organic carbon in an amount between 0.05 and 5 ml wet resin per liter of water. The preferred ion exchange resin is a magnetic resin. U.S. patent Publication No. US 2002/0121479 A1 also discloses use of particulate magnetic ion exchange resin dispersed in water in a water purification process. None of these publications disclose methods for using such particulate ion exchange resins for removal of inorganic ionic species down to acceptable levels, which are often in the range of parts-per-billion for drinking water. The MIEX® resin described in U.S. Pat. No. 5,900,146 has been used to remove dissolved organic carbon down to the parts-per-million level.
However, the removal of some toxic inorganic ionic species from water down to the parts-per-billion (ppb) level is necessary in order to provide high quality water suitable for distribution and consumption. For example, EPA standards currently require no more than 50 μg/L (50 ppb) arsenic in drinking water. This Maximum Contaminant Level (MCL) will be reduced to 10 μg/L in January 2006. MCLs for chromium and perchlorate are due to be set in the near future.
Removal of contaminating inorganic anions by ion exchange in the presence of competing ions such as sulfate, nitrate, bicarbonate and dissolved organic compounds present in the water has not heretofore been widely adopted primarily because the competing ions exhaust the resin before significant amounts of the target inorganic anions (e.g., arsenate) have been removed, or because unless the contact time between the water and resin is carefully calibrated and constantly readjusted to account for small concentration differences, there is a significant risk of breakthrough and chromatographic peaking events. Thus, frequent regeneration of the resin and the need for careful monitoring of the process can make removal of such contaminating ions by means of ion exchange resins too difficult to operate to be viable. When silicate is present as a competing ion, fouling of the ion exchange resin is a severe problem in inorganics removal by means of ion exchange columns. In such cases, it is believed that the resin particles become coated with polymerised silicate, leading to an impenetrable layer of solid material on and near the surface of the bed, resulting in the columns becoming inoperable for inorganic ionic species removal.
This invention fulfills the need for a water treatment process which can economically remove inorganic ionic species contaminants from water while eliminating breakthrough and chromatographic peaking.
All publications referred to in this patent application are incorporated by reference to the extent not inconsistent herewith for their teachings of known water processing steps, systems, and reagents.